In frequency modulation, which is one of the communication methods and in which communication is performed by causing an instantaneous frequency to be proportional to the modulation baseband, GFSK (Gaussian-filtered Frequency Shift Keying), in which a band is limited by applying a Gaussian filter to a modulated signal, and GMSK (Gaussian-filtered Minimum Shift Keying), in which minimum instantaneous frequencies whose shifts are orthogonal, are known as communication methods. These are also generally referred to as FSK modulation.
In radio communication, fading occurs due to multipath at the time of reception and this significantly reduces received power. If the received power is reduced due to fading, a problem arises in that the performance of demodulation of the received signal deteriorates. One technique to solve this problem is a diversity technique, in which a plurality of antennas are used so as to utilize the fact that the propagation paths of signals received by the antennas are different from each other. Various diversity techniques are present and one of them is an antenna selection diversity technique, in which one antenna is selected from among a plurality of antennas. With this antenna selection diversity technique, when a signal is transmitted or received, an antenna having excellent propagation path characteristics is selected to transmit or receive the signal, thereby improving the communication quality.
A diversity technique is disclosed in Patent Literature 1 in which the FSK demodulator selects an antenna having a larger RSSI value (received signal strength) of a received signal on the basis of the detection result of a preamble signal.